The United States is one of the largest producers of sulfur in the world. Much of the sulfur is produced as a by-product from the processing of gases containing hydrogen sulfide (H.sub.2 S) and/or sulfur dioxide (SO.sub.2), including natural gas, gas from crude oil production, and by-product gases from petroleum refining. Typically, in these and other plants constructed during the 1980's, the H.sub.2 S present in the gas stream that is fed into the process ("feed gas") is converted by the process to SO.sub.2, which is then converted by a catalytic process to elemental sulfur. This is normally accomplished by a number of well known processes, such as the Claus Sulfur process, the Wellman-Lord process, and the Stretford process.
In the Claus process, one-third of the H.sub.2 S in the feed gas is burned with stoichiometric amounts of air to produce SO.sub.2. The remaining H.sub.2 S then reacts with SO.sub.2 in a series of catalytic converters to form elemental sulfur. The residual SO.sub.2 contained in the feed gas after treatment ("tail gas") is normally vented to the atmosphere. As emission requirements have become more stringent, these plants have had to be retrofitted to meet Clean Air Act regulations of the 1990's. This retrofitting utilizes new technology which is either a modification of the original process or an add-on process. Due to the large volume of by-product sulfur being produced, the price of sulfur has declined steadily in the last five years from nearly $100 per ton to the current price of about $40 per ton. Consequently, new technology must be increasingly effective, in terms of SO.sub.2 removal efficiency, capital costs, and operating costs.
High SO.sub.2 emissions in the flue-gas of coal-fired power plants have also necessitated more effective measures to control and dispose of SO.sub.2. Some plants have adopted technology based on absorbing the SO.sub.2 in throwaway lime or limestone solutions or slurries. The extremely corrosive nature of the solutions and slurries of these processes have made their operating costs quite prohibitive. Furthermore, disposal of the solid waste generated by these processes has caused increasing environmental concerns.
In the limestone slurry systems, the SO.sub.2. is reacted with a lime or limestone slurry. The resulting slurry is then disposed of by land-farming or other means. The SO.sub.2 removal efficiency of these slurry systems is increased by the addition of pH buffers.
The use of either pure or impure dibasic acids as a pH-buffering additive in limestone-slurry systems is disclosed in a number of articles, including: Chi, "Using Byproducts: A Case Study," ChemTech, p. 308, May, 1990; Chang, et al., Effect of Organic Acid Additives on SO.sub.2 Absorption into CaO/CaCO.sub.3 Slurries," American Institute of Chemical Engineers Journal, Vol. 28, No. 2, p. 261, March, 1982 (the preferred pH range according to this article is 4-6); Chang, et al., "Testing and Commercialization of Byproduct Dibasic Acids as Buffer Additives for Limestone Flue Gas Desulfurization Systems," Journal of the Air Pollution and Control Association, Vol. 33, No. 10, p. 955, October, 1983; and, Lee, et al., "Oxidative Degradation of Organic Acid Conjugated with Sulfite Oxidation in Flue Gas Desulfurization: Products, Kinetics, and Mechanism," Environmental Science and Technology, Vol. 21, No. 3, p. 266-272, 1987. The use of various dibasic acids has been shown to improve absorption rates of the limestone slurries.
For those processes that absorb the sulfur gases into a liquid first, much of the prior art utilizes an organic solvent, which chemically reacts with the SO.sub.2 and H.sub.2 S. Examples of this type of process include Shell's "Sulferox Process", Hydrocarbon Processing's Gas Process Handbook, Gulf Publishing, 1992; and those disclosed in U.S. Pat. No. 3,832,454; U.S. Pat. No. 3,928,548; and U.S. Pat. No. 4,069,302.
All these processes that produce sulfur from sulfur gases are energy intensive and present hazards because of the use of organic solvents. Furthermore, most of these are not cost efficient, nor do they generally achieve essentially complete removal of the sulfur gases.
Some removal processes utilize solvent-based reactions, but then regenerate SO.sub.2 instead of sulfur. The SO.sub.2 released from the solvent can be dried, and liquified and sold for its chemical value. The market price for liquid SO.sub.2 has remained relatively steady at nearly $225 per ton in the last five years, which is far more than the selling price of sulfur. An example of a solvent-based SO.sub.2 generating technology is disclosed in U.S. Pat. No. 4,885,146. All solvent-based systems suffer from high expense and the dangers normally associated with use of solvents.
The produced liquid SO.sub.2 has a variety of industrial applications. Liquid SO.sub.2 has been known as a good solvent for the purification of lubricating oils and for increasing oil viscosity and paraffinity. It has also been used as a solubilizing agent of phosphates and dyes, as raw material to produce sulfuric acid and sulfolane, as an excellent polymer solvent, and for sulfonation with SO.sub.2. Liquid SO.sub.2 is usually manufactured by the sulfur-burning process and through recovery from metallurgical sources. Its availability from such sources, however, is subject to fluctuations in economic conditions and the state of labor relations in the metallurgy industry. Recovery of liquid SO.sub.2 from waste product of acid gas removal processes can certainly be a supplemental source to help stabilize the liquid SO.sub.2 supply to the industry.
To avoid the problems associated with solvent-based processes, some processes are aqueous-based. Such absorption liquids contain various components, mostly to enhance the absorption abilities of the liquid. For instance, numerous studies have shown that SO.sub.2 absorption is enhanced at certain pH ranges, though there are disagreements in the literature as to exactly what is the optimum pH range. Regardless, as SO.sub.2 is absorbed, the pH of the solution tends to be more acidic. Therefore, a buffering agent must be added to keep the pH in the proper range. U.S. Pat. No. 4,965,062 is an example of an aqueous-based system, and discloses a method for reacting H.sub.2 S to elemental sulfur. This aqueous-based process uses sulfite ions and an acetic acid--acid salt buffering system. This system requires H.sub.2 S recycle to properly work.
Various aqueous processes using a sodium citrate absorption solution have been in use since the early 1970's. Sodium citrate is a popular absorption product because it tends to buffer the absorption solution to keep it in the pH range of 3.5 to 5.5, where maximum absorption and desorption of SO.sub.2 can occur. The absorbed SO.sub.2 can either be reacted to produce elemental sulfur, or recovered unaltered. See Information Circulars 7774, 8540, 8793, 8806, and 8819, Bureau of Mines, United States Department of the Interior. Other similar processes that react the SO.sub.2 to elemental sulfur are disclosed in U.S. Pat. No. 4,048,293; U.S. Pat. No. 4,519,994; U.S. Pat. No. 3,983,225; and, U.S. Pat. No. 4,450,145.
Some processes concentrate and recover the SO.sub.2 unaltered, instead of reacting it to elemental sulfur. The recovered SO.sub.2 can then be used as feed for another process or liquified and sold, as discussed above. The citrate process discussed above can be used for absorption and desorption of SO.sub.2, as for example disclosed in U.S. Pat. No. 3,886,069.
Other similar aqueous processes are disclosed in the following; Bengtsson, "The Flakt-Boliden SO.sub.2 Recovery Process", Chemistry in Canada, January, 1981; Aqueous Absorbents for Stack Gas Desulfurization by Absorption/Stripping, Electric Power Research Institute, CS-3185, July 1983; "The Recovery of Sulfur from Smelter Gases," Journal of the Society of Chemical Industry, Vol. 56, p. 139, May, 1937; U.S. Pat. No. 4,181,506; "Union Carbide claims 99t Effectiveness for Flue Gas Scrubber", Vol. 89, No. 46, Oil and Gas Journal, Nov. 18, 1991; Electric Power Research Institute Report, CS-3228, Final Report, October 1983; Erga, "A New Regenerable Process for the Recovery of SO.sub.2 ", Chemical Engineering Technology, Vol. 11, p. 402-407, 1988; Erga, "SO.sub.2 Recovery by Means of Adipic Acid Buffers", Industrial Chemical Engineering Fundamentals, Vol. 25, p. 692-695, 1986; Goar, "Today's Sulfur Recovery Processes", Hydrocarbon Processing, Vol. 47, No. 9, p. 249-252, 1968; Kumazawa, "Simultaneous Removal of NO and SO.sub.2 by Absorption into Aqueous Mixed Solutions," American Institute of Chemical Engineers Journal, Vol. 34, No. 7, pp. 1215-1220; Johnstone, et al., "Recovery of Sulfur Dioxide from Waste Gases", Industrial and Engineering Chemistry, p. 101-109, January, 1938.
All of the processes for SO.sub.2 removal discussed above, whether aqueous or non-aqueous, suffer from some related problems. First, most of these processes cannot reduce the amount of SO.sub.2 in the effluent gas stream to below 100 ppm. One key reason for this lack of efficiency is the lack of sufficient solubility of SO.sub.2 in the absorption liquid. Another reason is the lack of efficient contacting between the SO.sub.2 and the absorption liquid. Considering current and near-future EPA regulations on emissions of SO.sub.2, reduction of the amount of SO.sub.2 remaining in the stream is a necessity and reduction to near zero may soon be required.
Second, almost all of these systems are not cost effective and in fact they lose money. That is, they cost more to operate than is made by selling the recovered sulfur or SO.sub.2. These systems are expensive to operate because of high energy consumption, as most require elevated temperatures. Also, the systems that absorb and regenerate SO.sub.2 tend to rapidly build up byproducts in the absorption liquid and these byproducts must be removed to maintain the efficiency of the processes. Also, many of the components used in the absorption liquid are relatively expensive, hazardous to use, or corrosive, and thus require more expensive handling equipment.
Third, many of these processes do not tolerate variations in the incoming feed stream very well, and are easily upset.
A more cost efficient SO.sub.2 removal process developed to date is the Aquaclaus process. In the Aquaclaus process, the SO.sub.2 is absorbed in an aqueous solution containing phosphoric acid (H.sub.3 PO.sub.4) and sodium carbonate (Na.sub.2 CO.sub.3), with the active chemical species being sodium phosphate. The absorbed SO.sub.2 is reacted with H.sub.2 S to generate elemental sulfur. The sulfur is separated from the absorption liquid, and the absorption liquid is recycled for use again. For a basic discussion of this process, see Hayford, "Process Cleans Tail Gases," Hydrocarbon Processing, Vol. 52, No. 10, p. 95-96, 1973. One of the desirable aspects of this process is its resistance to upsets due to variations in the feed stream, as well as its high efficiency of removal.
However, what is needed in the marketplace and what is embodied in the present invention is a SO.sub.2 removal process that is profitable to operate and removes virtually all the SO.sub.2 from the feed stream in one pass, regardless of variations in the feed stream.